Recently, researches on hybrid type construction equipment, which improves fuel efficiency by storing surplus power of an engine in a battery, and supplying power from the battery to the engine that does not have sufficient power so as to cope with a rapid increase in oil price, are being actively conducted.
A system, which uses the engine and an electric motor as a common power source as described above, and has an electrical energy storage apparatus, is referred to as a hybrid system. For example, as the hybrid system, there is a hybrid system for heavy equipment such as a hybrid vehicle, and an excavator.
A general excavator system performs an operation of driving a boom, an arm, and a bucket, which are final loads, and swing or travelling by using an engine as a power source through a medium, that is, hydraulic pressure. On the contrary, in the case of a hybrid excavator system, two motors and an electricity storage apparatus are additionally installed in a general excavator, thereby improving overall efficiency of the excavator system. Main components added to the hybrid excavator system include a motor, an energy storage apparatus, an inverter, and a converter. Here, the energy storage apparatus includes a battery and an ultra-capacitor (UC).
In general, a direct current terminal voltage necessary for power conversion of the inverter and the converter in the hybrid excavator is varied by unbalance of an output of an engine auxiliary motor and an output of a swing motor. Accordingly, a converter for the energy storage apparatus is applied in order to constantly maintain a direct current terminal voltage.
In this case, a reference voltage for determining a charging/discharging operation of the converter, that is, an instruction voltage of the direct current terminal voltage control unit has a uniform value regardless of an operation of construction machinery. Further, the instruction voltage is set with a value large enough to encompass all of the speeds and torque regions at and in which the engine auxiliary motor and the swing motor are operable.
A method of controlling a direct current terminal voltage of the hybrid excavator will be described below. In general, an engine auxiliary inverter, an inverter for swinging, and a converter for an energy storage apparatus connected to the direct current terminal are formed of a semiconductor switch. Energy loss generated during the driving of the semiconductor switch is proportional to an output current, a switching frequency, and a direct current terminal voltage. A size of the output current is determined under a load condition. The switching frequency is mainly determined by inductance of an output terminal and a size of a current pulsatory motion. The direct current terminal voltage is mainly designed to be a maximum voltage capable of encompassing all of the speeds and the torque regions, at and in which the engine auxiliary motor and the swing motor are operable, within a voltage range allowed by the semiconductor switch.
As described above, when the engine auxiliary motor or the swing motor are driven at a low speed in the hybrid excavator, an excessively large direct current terminal voltage is maintained regardless of driving of the inverter, so that a voltage loss is unnecessarily is generated. Particularly, there is a large problem in a voltage loss in a low speed and high torque region.
The discussion above is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter.